1. Field of the Invention
This invention relates to a method for the production of a microcapsule (MC) type conductive filler and more particularly to a method for coating the surface of minute conductive particles with an insulating polymer and to an MC type adhesive agent having dispersed in an adhesive agent the coated MC type conductive filler.
2. Description of the Related Art
In the conventional method of adhesion, the adhesion effected by soft soldering or welding where the interface produced by this adhesion requires conductivity. The conventional method is effectively applicable only to a limited number of materials because of the heat factor. In contrast, the organic-inorganic composite conductive adhesive agent that is composed of a binder using a synthetic resin as a main component thereof and a conductive filler using a metal powder as a main component thereof finds utility in a wide variety of applications that involving different kinds of materials subjected to adhesion. This adhesive agent, therefore, is an indispensable medium for conductive adhesion of plastic substances (such as epoxy and phenol resins) that do not adhere by soft soldering, for adhesion of NESA glass used in liquid crystal display devices, for adhesion of phosphor bronze with a carbon brush used in micrometers, and for adhesion of lead wires as in quartz oscillators and sdc meters, for example.
Particularly, in the semiconductor industry, which has been enjoying significant growth recently, IC's and LSI's of increasingly high quality have been developed and mass produced. For the adhesion of these semiconductor chips (silicon wafers) to lead frames, though the method involving to an Au--Sn eutectic once prevailed, conductive adhesive agents formed by kneading an epoxy resin with silver powder now have multiple applications utility owing to their ability to lower cost and enhance productivity.
As a resin binder for conductive adhesive agents, while epoxy resin is generally used, polyimide type, phenol type, and polyester type resins are also used, though only partially. As a conductive filler, minute particles of such metals as gold, silver, and copper and amorphous carbon and graphite powder are generally used as well as metal oxides, though only partially. Silver powder is preferably used over the conductive fillers cited above because it is inexpensive, reliable and effective.
The conductive adhesive agent is advantageous in various respects compared with conventional applications such as soft soldering and welding though it is not perfectly free from fault. When this conductive adhesive agent is used between an LSI chip and patterns for mounting component parts, for example, an increase in the amount of minute conductive particles that are incorporated in the conductive adhesive agent lowers insulation resistance as illustrated in FIG. 1 and increases the possibility of adjacent patterns forming electric continuity. A reduction in the amount of minute conductive particles reduces the electric continuity between the LSI and the patterns. Data indicate the necessity for rigidly controlling the amount of minute conductive particles to be used in the conductive adhesive agent. And at the same time, reveal the fact that the minute conductive particles cannot be used in large amounts.
It is believed possible that this problem can be solved by a procedure that comprises preparing an MC type conductive adhesive agent having dispersed in an adhesive agent, an MC type conductive filler formed by coating the surface of minute conductive particles with an insulating polymer, applying the MC type conductive adhesive agent to the entire surface of the substrate of an IC or LSI chip, exerting pressure to bear on the interface between the chip and patterns deposited thereon, thereby rupturing the coating layer of the capsules and establishing electric continuity between the chip and the patterns, and meanwhile allowing the encapsulated minute conductive particles interposed between the adjacent patterns to remain intact and continue to insulate these patterns from one another.
The insulating resins that are usable for coating the surface of minute conductive particles include thermoplastic resins and thermosetting resins as classified by kind. In terms of resistance to moisture absorption and electric insulating properties, thermosetting resins definitely excel thermoplastic resins. Since thermocompression bonding of a chip to a substrate is generally carried out at an elevated temperature of at least 170.degree. C., the insulating resin to be used is required to be stable enough to resist this elevated temperature though few thermoplastic resins can endure this temperature. In contrast, most thermosetting resins can tolerate temperatures in the neighborhood of 200.degree. C.
For use as an insulating resin in the MC type conductive filler, thermosetting resins that are advantageous in various respects over thermoplastic resins are suitable.
For the application of an insulating resin coating to the surface of minute conductive particles, however, the procedure that involves dissolving the resin in a solvent, spraying the solution on the surface of the minute conductive particles, and drying the applied coating of the solution is predominant though since thermosetting resins are insoluble in solvents, this procedure applied conventionally is difficult and the application of a thermosetting resin coating to the surface of minute conductive particles, therefore, necessitates development of a novel coating procedure.
The prior techniques pertaining to the MC type conductive adhesive agent have been disclosed by Japanese Unexamined Patent Publications No. 176,139/1987, No. 76,215/1987, No. 47,943/1988, No. 54,796/1988, No. 103,874/1990, and No. 103,875/1990, for example.
First, the disclosures of Japanese Unexamined Patent Publications No. 176,139/1987, No. 76,215/1987, No. 47,943/1988, and No. 54,796/1988 will be described. These patent publications disclose, as conductive adhesive agents, those produced by forming an intermediate conductive layer on spherical cores of resin and coating the intermediate layer with a surface layer of an insulating thermoplastic resin and those produced by coating the surface of minute spherical conductive particles with an insulating thermoplastic resin. Actual mounting of a chip on a substrate for a printed circuit by using such a conductive adhesive agent is attained by a procedure that comprises applying the conductive adhesive agent to the substrate and thermocompression bonding the chip to the substrate so that the intermediate layer or the minute conductive particles will discharge a conductive function and the insulating thermoplastic resin an adhesive function and an insulating function. The techniques disclosed by these patent publications differ from the method using the MC type conductive adhesive agent of the present invention and these patent publications do not mention using a thermosetting resin as an insulating resin for coating the surface of the minute conductive particles.
Now, the disclosure of Japanese Unexamined Patent Publication No. 103,874/1990 will be described below. The invention of this patent publication pertains to an MC type conductive adhesive agent produced by dispersing in a film of an insulating adhesive agent serving as a binder an MC type conductive filler having minute conductive particles coated with an insulating thermoplastic resin or thermosetting resin. Conductive union of two given members using this MC type conductive adhesive agent is accomplished by depositing this adhesive agent on the two members and pressing the two members against each other while being heated state. Thus, in the part expected to form electric continuity, the impact of the pressure exerted as described above ruptures the insulating resin layer of the MC filler and establishes the desired electric continuity, whereas in the part requiring insulation, the MC type conductive filler is allowed to remain intact and, therefore, retain stable insulation. Incidentally, this MC type conductive filler is manufactured by plasma polymerization or plasma CVD polymerization and there are times when the insulating film of the MC type filler may be formed of a thermosetting resin. The number of kinds of thermosetting resins that can be manufactured by the plasma polymerization and the plasma CVD polymerization is very small because the number of kinds of gases usable for injection during the polymerization is not large. Further in accordance with this method of plasma polymerization or plasma CVD polymerization, the cost is sufficiently high to render the manufacturing thereof impracticable and productivity is inferior because the amount of MC type filler to be manufactured is small.
The disclosure of Japanese Unexamined Patent Publication No. 103,875/1990 will be described below. The invention of this patent publication pertains to the use of an MC type conductive adhesive agent produced by coating minute conductive particles with an insulating thermoplastic resin or thermosetting resin. Actual mounting of a chip on a substrate for a printed circuit using this MC type conductive adhesive agent is attained by applying the conductive adhesive agent to the substrate and thermocompression bonding the chip to the substrate, with the intermediate layer or the minute conductive particles discharging a conductive function and the insulating resin on the surface of the minute conductive particles an adhesive function and an insulating function. Incidentally, this MC type conductive filler is manufactured by either plasma polymerization or plasma CVD polymerization. Thus, these prior techniques are described as allowing what is formed by coating the surface of minute conductive particles with a thermosetting resin. In spite of these disclosures, thermosetting resins should be unusable for the purpose of coating because they do not melt with heat and, therefore, are incapable of functioning as an adhesive. Even if a thermosetting resin is used, the method of manufacturing the MC type conductive filler entails a serious drawback as pointed out in Japanese Unexamined Patent Publication No. 103,874/1990.
Practically all the prior techniques pertaining to the manufacture of an MC type conductive filler or the conductive adhesive agent using this filler invariably use a thermoplastic resin. Even when the patent publications mention usability of a thermosetting resin, methods of manufacturing using such a thermosetting resin are not disclosed with sufficient specificity or are devoid of practicability and thus, these methods cannot be actually used.